Tendons are specialized tissues that connect bone to muscle transmitting forces generated by muscles to allow body movement. Tendon injuries due to trauma and overuse are a common clinical problem. Damaged tendon heals very slowly and rarely attains the structural integrity and mechanical strength of normal undamaged tendon. Sharma, P. & Maffulli, N., J. Musculoskelet. Neuronal Interact., 6(18):1-90 (2006). The development of new treatment options for injured tendons has been hindered because of our limited understanding of the basic tendon biology. Id.
The primary unit of tendon is comprised of collagen fibrils that cross-link to each other in a staggered fashion to form fibers. Tendon cells reside in between long parallel chains of the collagen fibrils and synthesize a unique extracellular matrix (ECM) that contains primarily collagens, large proteoglycans, and small leucine-rich proteoglycans (SLRPs), which function as lubricators and as organizers for collagen fibril assembly. Despite the large presence of the extracellular matrix in tendon, very little is known about its role in regulating the function of cells that reside within it. A better understanding of the mechanisms that regulate the function and the differentiation of tendon cells is essential to developing new treatments for tendinopathy such as tendon rupture or ectopic ossification due to injury from overuse or from trauma.
Moreover, many types of tissues are subject to the effects of aging, and become deficient over time. One effect of aging is the loss of elasticity in tissue. This affects the appearance of the tissue and its function. While various lotions and medicaments are available for treatment of the effects of aging, none of them are particularly effective. For example, lotions and creams containing collagen are available but without the ability to form functional collagen fibrils, such collagen creams are not particularly effective for improving the elasticity of skin and other tissues. Thus, the effects of aging and age-related tissue problems are often related to a deficit of functional cell types. Lower cell populations and changing gene expression patterns can alter the elasticity, resilience, recovery from injury, cell proliferation, cell differentiation, signaling pathways, feedback mechanisms, and tissue homeostasis, amongst other physiological processes.
Accordingly, a need exists for more insight into tendon cells and methods for regulating tendon cell function, growth and differentiation. Also a need exists for improving the elasticity, resilience, and recovery from injury in aging tissues, as well as repairing injured or diseased tissues, especially elastic tissues such as tendons.